Wetting resistant materials and articles made therewith

ABSTRACT

Ceramic materials with relatively high resistance to wetting by various liquids, such as water, are presented, along with articles made with these materials, methods for making these articles and materials, and methods for protecting articles using coatings made from these materials. One particular embodiment is an article that comprises a coating having a surface connected porosity content of up to about 5 percent by volume. The coating comprises a material that comprises a primary oxide and a secondary oxide, wherein (i) the primary oxide comprises a cation selected from the group consisting of cerium, praseodymium, terbium, and hafnium, and (ii) the secondary oxide comprises a cation selected from the group consisting of the rare earth elements, yttrium, and scandium.

BACKGROUND

This invention relates to wetting resistant materials. Moreparticularly, this invention relates to articles that include coatingsof wetting resistant materials.

The “liquid wettability”, or “wettability,” of a solid surface isdetermined by observing the nature of the interaction occurring betweenthe surface and a drop of a given liquid disposed on the surface. A highdegree of wetting results in a relatively low solid-liquid contact angleand large areas of liquid-solid contact; this state is desirable inapplications where a considerable amount of interaction between the twosurfaces is beneficial, such as, for example, adhesive and coatingapplications. By way of example, so-called “hydrophilic” materials haverelatively high wettability in the presence of water, resulting in ahigh degree of “sheeting” of the water over the solid surface.Conversely, for applications requiring low solid-liquid interaction, thewettability is generally kept as low as possible in order to promote theformation of liquid drops having high contact angle and thus minimalcontact area with the solid surface. “Hydrophobic” materials haverelatively low water wettability (contact angle generally at or above 90degrees); so-called “superhydrophobic” materials (often described ashaving a contact angle greater than 120 degrees) have even lower waterwettability, where the liquid forms nearly spherical drops that in manycases easily roll off of the surface at the slightest disturbance.

Heat transfer equipment, such as condensers, provide one example of anapplication where the maintenance of surface water as droplets ratherthan as a film is important. Two alternate mechanisms may govern acondensation process. In most cases, the condensing liquid(“condensate”) forms a film covering the entire surface; this mechanismis known as filmwise condensation. The film provides a considerableresistance to heat transfer between the vapor and the surface, and thisresistance increases as the film thickness increases. In other cases,the condensate forms as drops on the surface, which grow on the surface,coalesce with other drops, and are shed from the surface under theaction of gravity or aerodynamic forces, leaving freshly exposed surfaceupon which new drops may form. This so-called “dropwise” condensationresults in considerably higher heat transfer rates than filmwisecondensation, but dropwise condensation is generally an unstablecondition that often becomes replaced by filmwise condensation overtime. Efforts to stabilize and promote dropwise condensation overfilmwise condensation as a heat transfer mechanism in practical systemshave often required the incorporation of additives to the condensingmedium to reduce the tendency of the condensate to wet (i.e., form afilm on) the surface, or the use of low-surface energy polymer filmsapplied to the surface to reduce film formation. These approaches havedrawbacks in that the use of additives may not be practical in manyapplications, and the use of polymer films may insert significantthermal resistance between the surface and the vapor. Polymer films mayalso suffer from low adhesion and durability in many aggressiveindustrial environments.

Texturing or roughening the surface can change the contact angle ofwater on a surface. A texture that increases the tortuosity of thesurface but maintains the contact between water droplet and the surfacewill increase the contact angle of a hydrophobic material and decreasethe contact angel of a hydrophilic material. In contrast, if a textureis imparted that maintains regions of air beneath a water droplet, thesurface will become more hydrophobic. Even an intrinsically hydrophilicsurface can exhibit hydrophobic behavior if the surface is textured tomaintain a sufficiently high fraction of air beneath the water drop.However, for applications requiring highly hydrophobic orsuperhydrophobic behavior, it is generally more desirable in practice totexture a hydrophobic surface than to texture a hydrophilic surface. Anintrinsically hydrophobic surface usually provides the potential for ahigher effective contact angle after texturing than an intrinsicallyhydrophilic surface, and generally provides for a higher level ofwetting resistance even if the surface texturing becomes less effectiveover time as the texture wears away.

Relatively little is known about the intrinsic hydrophobicity of broadclasses of materials. In general, most of the materials known to have acontact angle with water of greater than 90 degrees are polymers such astetrafluoroethylene, silanes, waxes, polyethylene, and propylene.Unfortunately, polymers have limitations in temperature and durabilitythat can limit their application, because many practical surfaces thatwould benefit from low wettability properties are subject in service tohigh temperatures, erosion, or harsh chemicals.

Ceramic materials are typically superior to polymers in many aspectsrelated to durability. Of the ceramic materials, oxide ceramics areparticularly useful because they are highly manufacturable, often havehigh environmental resistance, and can have good mechanical properties.Unfortunately, there are virtually no known oxide ceramics that arehydrophobic. A notable exception is silicalite, a zeolitic polymorph ofSiO2 [E. M. Flanigen, J. M. Bennett, R. W. Grose, J. P. Cohen, R. L.Patton, R. M. Kirchner, and J. V. Smith, “Silicalite, a new hydrophobiccrystalline silica molecular sieve,” Nature, v. 271, 512 (1978)]. Forthat material the specific crystal structure is highly important becauseamorphous SiO2 has a very low, hydrophilic wetting angle. However, thesynthesis conditions required to form zeolite crystals can limit therange of applicability of those materials as hydrophobic surfaces andthe porosity of zeolite crystals makes them less desirable forapplications requiring durability.

Therefore, there remains a need in the art for oxide ceramics that havelower liquid wettability than conventional oxides, promote stabledropwise condensation, are stable at elevated temperatures, are amenableto coating processing, and have good mechanical properties. There isalso a need for articles coated with these wetting resistant oxideceramics.

BRIEF DESCRIPTION

Embodiments of the present invention are provided to meet these andother needs. One embodiment is a material comprising a primary oxide anda secondary oxide. The primary oxide comprises cerium and hafnium. Thesecondary oxide comprises a secondary oxide cation selected from thegroup consisting of the rare earth elements, yttrium, and scandium.

Another embodiment is a material comprising a primary oxide and asecondary oxide. The primary oxide comprises cerium or hafnium. Thesecondary oxide comprises (i) praseodymium or ytterbium, and (ii)another cation selected from the group consisting of the rare earthelements, yttrium, and scandium.

Further embodiments include articles and coatings that include any ofthe materials described herein. As an example, one particular embodimentis an article that comprises a coating having a surface connectedporosity content of up to about 5 percent by volume. The coatingcomprises a material that comprises a primary oxide and a secondaryoxide, wherein (i) the primary oxide comprises a cation selected fromthe group consisting of cerium, praseodymium, terbium, and hafnium, and(ii) the secondary oxide comprises a cation selected from the groupconsisting of the rare earth elements, yttrium, and scandium.

Further embodiments include methods for protecting articles. The methodincludes depositing a coating on a substrate, wherein the coatingcomprises any of the materials or coatings described herein.

DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 is a data plot showing static water contact angle as a functionof material composition for oxides comprising lanthanum, cerium, andhafnium;

FIG. 2 is a schematic of an exemplary embodiment of the presentinvention;

FIG. 3 data plot showing static water contact angle as a function ofmaterial composition for oxides comprising cerium and lanthanum;

FIG. 4 is a data plot showing static water contact angle as a functionof material composition for oxides comprising cerium and praseodymium;

FIG. 5 is a data plot showing static water contact angle as a functionof material composition for oxides comprising cerium and neodymium; and

FIG. 6 is a schematic of an exemplary embodiment of the presentinvention.

DETAILED DESCRIPTION

Embodiments of the present invention are based upon the discovery by theinventors of a class of oxide ceramics that shows certain surprisingproperties. First, they tend to have significantly lower waterwettability than commonly known engineering oxides. Some compositionsare intrinsically hydrophobic. Moreover, some compositions, even thosenot intrinsically hydrophobic, have demonstrated the ability to maintainstable dropwise water condensation, making them intriguing candidatesfor use in heat transfer applications, for instance. Without being boundby theory, it is suspected that this behavior is related to the natureof the oxygen-cation bonding occurring within the crystal structure ofthe oxide.

Embodiments of the present invention include certain materialcompositions. Other embodiments include coatings and articles thatinclude these compositions. These compositions may exist in any form,such as, for example, powders, coatings, and ingots. The materialsdescribed herein are generally a mixture or a compound of multipleoxides. Throughout this description, the composition of the material maybe described in terms of its component oxides, such as, for example,cerium oxide (CeO₂) and lanthanum oxide (La₂O₃), even if these componentoxides are technically not present in the material due to interactionssuch as phase transformations and chemical reactions. This notation isconsistent with that commonly used in the art, where, for example, acompound such as lanthanum cerate may be interchangeably denoted asLa₂O₃.2CeO₂, LaO_(1.5).CeO₂, or La₂Ce₂O₇.

The material used in certain embodiments of the present invention has acubic crystal structure of the fluorite type. Cerium oxide is an exampleof a ceramic oxide with this crystal structure. As described below, thepresent inventors have, in some cases, observed remarkable wettingcharacteristics for materials having this crystal structure, though itis not assumed at this time that the presence of this crystal structureis a necessary condition to develop such characteristics.

One of the oxides of the material is referred to herein as the “primaryoxide.” The primary oxide has a cation having at least some tendency totetravalent behavior. The tetravalent quality of the primary oxidecation is believed to play a role in stabilizing the material in thepresence of water. Another of the component oxides of the material isreferred to herein as the “secondary oxide.” In certain embodiments, thesecondary oxide is selected so that it has a lower intrinsic surfaceenergy than the primary oxide. The lower “intrinsic” surface energygenerally is manifested by a higher contact angle for a reference liquid(such as, for example, water) on a surface of pure secondary oxiderelative to the contact angle for that liquid on a surface of pureprimary oxide. Examples of cations whose oxides have shown asurprisingly low surface energy (high reference liquid contact angle)include lanthanum, praseodymium, ytterbium, and neodymium. However, manyof the secondary oxides are at least somewhat hygroscopic, making them,when used by themselves, unsuitable for many practical applicationsinvolving at least incidental contact with moisture. This limitation wasfound to be mitigated, as described above, by the addition of primaryoxide in accordance with embodiments described herein, which additionappears to stabilize the material in the presence of water and thuspresents opportunities for use of the material in practical engineeringapplications.

It will be appreciated that where materials and articles are describedherein as “comprising” or “including” one or more components, the scopeof the description includes, without limitation, materials made only ofthe stated components; materials made of the stated components andincluding other components that do not materially affect the wettabilityof the material; and materials including the stated components but notexcluding other components. Moreover, where lists of alternatives areprovided, the alternatives are not meant to be exclusive; one or more ofthe alternatives may be selected, except where otherwise explicitlystated.

In one embodiment, a material includes a primary oxide that comprisescations (“primary oxide cations”) of cerium and hafnium. In someembodiments, a molar ratio of cerium cations to primary oxide cations isin the range from about 0.01 to about 0.99, and in certain embodimentsthis range is from about 0.1 to about 0.9. The material also includes asecondary oxide that comprises a cation (“secondary oxide cation”)selected from the group consisting of the rare earth elements, yttrium,and scandium; in particular embodiments the secondary oxide cation maybe lanthanum, praseodymium, ytterbium, or neodymium. In certainembodiments, a molar ratio of the primary oxide cations to total cationspresent in the material is in the range from about 0.1 to about 0.95; insome embodiments, this range is from about 0.25 to about 0.9; and inparticular embodiments, the molar ratio is about 0.5. FIG. 1demonstrates an example of the performance of such a material, where thesecondary oxide was lanthanum oxide, and the primary oxide includedcerium and hafnium in various relative proportions, with the overallratio of primary oxide cations to total cations present in the materialwas 0.5. Remarkably, although none of the combination materials testedhad a contact angle of 90 degrees or greater, all compositionsdemonstrated dropwise water condensation behavior.

In another embodiment, a material includes a primary oxide comprising aprimary oxide cation of cerium or hafnium. In some embodiments, themolar ratio of the primary oxide cations to total cations present in thematerial is within any of the corresponding ranges provided for thepreviously described material. The material further includes a secondaryoxide that includes (i) a first secondary oxide cation comprisingpraseodymium or ytterbium, and (ii) a second secondary oxide cation thatcomprises a rare earth element, yttrium, or scandium. In certainembodiments, a molar ratio of first secondary oxide cations to totalsecondary oxide cations is in the range from about 0.01 to about 0.99;in certain embodiments this range is from about 0.05 to about 0.95; andin particular embodiments this range is from about 0.1 to about 0.90. Inspecific embodiments, the secondary oxide comprises praseodymium andlanthanum, and in some embodiments the secondary oxide comprisesytterbium and lanthanum.

Further embodiments of the present invention, as illustrated in FIG. 2,include an article 200 comprising a coating 202, where the coating 202comprises the materials described herein. In some embodiments, thisoxide material (“the material”) makes up greater than about 50 percentof the coating volume. In certain embodiments, the material makes upgreater than about 75 percent of the coating volume, and in someembodiments the material makes up substantially all of the coatingvolume (save for incidental impurities and void space). In someembodiments, this coating 202 has a low level of surface connectedporosity, such as up to about 5 percent by volume. In certainembodiments, the surface connected porosity is even lower, such as lowerthan 2 percent, lower than 1 percent, lower than 0.5 percent, or lowerthan 0.1 percent (all percentages by volume). In some embodiments, thecoating 202 is made of material that is substantially theoreticallydense. A low content of surface connected porosity may inhibit theabsorption of water into a pore network, thereby keeping liquid at thesurface of the article. Even a surface made of highly hydrophobicmaterial, for instance, may absorb water if the amount of open porosityis unduly high, thereby rendering the surface ineffective as a barrierto water.

In some embodiments, the article described above further comprises asubstrate 204, such as a metal substrate, for example, upon which theaforementioned coating 202 is disposed. Examples of metal substratesinclude metals and alloys made with aluminum, steel, stainless steel,nickel, copper, or titanium. In particular, common engineering alloyssuch as 306 stainless steel, 316 stainless steel, 403 stainless steel,422 stainless steel, Custom 450 stainless steel, commercially puretitanium, Ti-4V-6A1, and 70Cu-30Ni are suitable substrates.

Various intermediate coatings (not shown) may be applied for any reason,such as to achieve desired levels of adhesion between substrate andcoating, depending on the nature of the materials involved and theselected methods for processing the materials. Such variations generallyare within the knowledge of one skilled in the art. Thickness of thecoating will depend upon the nature of the environment and theapplication envisioned for the article. For example, in a heat exchangerapplication, the coating is typically designed to minimize thermalresistance between the environment and the substrate while achieving apractical service lifetime. Determination of the coating thickness for agiven application is within the knowledge of one skilled in the art.

In some embodiments the material, whether embodied in a coating orfreestanding object, has a low level of overall porosity, such as lowerthan about 5 percent by volume. In certain embodiments, the overallporosity of the material is even lower, such as lower than about 1percent. In some embodiments, the material is substantiallytheoretically dense throughout. The overall porosity of the material,like the thickness of the coating described above, plays a role indetermining the thermal resistance of the article: higher porositytypically results in high thermal resistance. Thus, maintaining a lowoverall porosity may be important in embodiments where low thermalresistance is desirable.

Any manufacturing method useful for fabrication and/or deposition ofceramic oxide materials may be used for fabricating the materials andarticles described herein. Accordingly, embodiments of the presentinvention include a method for protecting an article from aliquid-containing environment, comprising applying a coating 202 to asubstrate 204, where the coating 202 comprises any of the materials orcoatings described herein. Examples of well-known processes capable ofmaking ceramic oxide materials include powder processing, sol-gelprocessing, chemical vapor deposition and physical vapor deposition. Inpowder processing methods, a ceramic article is formed from ceramicparticles using a method such as pressing, tape casting, tapecalendaring or screen printing, and then consolidating and densifyingthe powders using a sintering process. Sol-gel processing methodsprovide a ceramic precursor in liquid form to a substrate after whichthe ceramic material is substantially formed through chemical reactionssuch as hyrdrolyzation and polymerization, and subsequently heat treatedto produce and densify the ceramic material. Chemical vapor depositionmethods involve providing gaseous precursor molecules to a heatedsubstrate to form a ceramic article and include atmospheric pressurechemical vapor deposition, low-pressure chemical vapor deposition,metal-organic chemical vapor deposition and plasma enhanced chemicalvapor deposition. Physical vapor deposition processes produce a vapor ofmaterial from solid precursors and supply the vapor to a substrate toform a ceramic article. Physical vapor deposition processes includesputtering, evaporation, and laser deposition. In the case of bulkceramic articles, the substrate is used to form the ceramic body in theform of a crucible, die or mandrel and subsequently removed. In the caseof ceramic coatings, the ceramic article remains attached to thesubstrate. The processing methods can be selected and tailored by apractitioner skilled in the art to produce the desired control ofchemical composition and density of the ceramic oxide articles.

In some embodiments, the surface of the material, e.g. a coating 202,further comprises a surface texture 206 to further improve thewetting-resistant properties of the article. A surface texture 206comprises features 208 disposed at the surface; examples of suchfeatures include, without limitation, elevations (such as cylindricalposts, rectangular prisms, pyramidal prisms, dendrites, nanorods,nanotubes, particle fragments, abrasion marks, and the like); anddepressions (such as holes, wells, and the like). In some embodiments,the surface texture serves to increase the tortuosity of the surface,which increases the contact angle of a hydrophobic material. In otherembodiments, the features are sized and configured to create pockets ofair between a drop of liquid and the surface, which can reduce theeffective surface energy and produce a higher contact angle than wouldbe expected for a smooth surface. Examples of such textures and methodsfor generating them are described in commonly owned U.S. patentapplication Ser. Nos. 11/497,096; 11/487,023; and 11/497,720; which areincorporated by reference herein in their entireties.

One particular embodiment of the present invention is an article 200.The article comprises a coating 202 that has the low level of surfaceporosity as described previously. This coating 202 comprises a materialthat includes primary and secondary oxides. In some embodiments, thematerial is any of those described above. In another embodiment, theprimary oxide comprises a cation selected from the group consisting ofcerium, praseodymium, terbium, and hafnium; and the secondary oxide,which, as described previously, has a lower intrinsic surface energythan the primary oxide, comprises a cation selected from the groupconsisting of the rare earth elements, yttrium, and scandium. Inparticular embodiments, the primary oxide comprises a cation selectedfrom the group consisting of cerium and hafnium. Moreover, in someembodiments the secondary oxide cation includes lanthanum, praseodymium,or neodymium. The descriptions above regarding embodiments that includesubstrates coated with the material, or the use of textured surfaces206, are also applicable for this embodiment.

In one set of embodiments, the primary oxide comprises cerium oxide.Cerium oxide may be combined with a secondary oxide to form a stableoxide material having desirable properties as described above. In oneexample, the secondary oxide comprises lanthanum oxide. In certainembodiments using this lanthanum-bearing material, the cerium cationmakes up at least about 25 molar percent of the cations present in thematerial; and in particular embodiments cerium cation makes from about45 molar percent to about 55 molar percent of the cations present in thematerial. FIG. 3 shows contact angle measurements made for a range ofbinary combinations of cerium oxide and lanthanum oxide. Remarkably, allcompositions tested produced stable dropwise water condensation, whilepure cerium oxide produced filmwise condensation.

In another example, the primary oxide comprises cerium oxide, and thesecondary oxide comprises praseodymium oxide. In certain embodiments,cerium cation makes up at least about 7 molar percent of the cationspresent in the material. In particular embodiments cerium cation makesup a molar percentage of the cations present in the material in therange from about 7 percent to about 60 percent, in which range thehighest water contact angles were observed. FIG. 4 shows contact anglemeasurements made for a range of binary combinations of cerium oxide andpraseodymium oxide. Again, despite the fact that cerium oxide itselfproduced filmwise water condensation, all of the ceriumoxide/praseodymium oxide combination materials tested surprisinglyproduced stable dropwise condensation.

In another example, the primary oxide comprises cerium oxide, and thesecondary oxide comprises neodymium oxide. In certain embodiments, thecerium cation makes up at least about 20 molar percent of the cationspresent in the material. In particular embodiments cerium cation makesup a molar percentage of the cations present in the material in therange from about 20 percent to about 60 percent, in which range thehighest water contact angles were observed. FIG. 5 shows contact anglemeasurements made for a range of binary combinations of cerium oxide andneodymium oxide. Again, surprisingly, all of the neodymium oxide/ceriumoxide combination materials tested produced dropwise water condensation.

Although certain examples described above highlighted materials made ofbinary combinations of a single primary oxide with a single secondaryoxide, it will be appreciated that embodiments of the present inventioninclude those in which one or both of the primary oxide and thesecondary oxide are made up of more than one oxide component. Forexample, in some embodiments the primary oxide comprises more than oneoxide, and each of these oxides is an oxide that includes as a cationcerium, praseodymium, terbium, or hafnium. In one example, the molarratio of primary oxide cation to secondary oxide cation is about 1,meaning that the material comprises an equal mole fraction of primaryoxide cation and secondary oxide cation. For instance, the material mayhave a composition denoted by the formula S₂Z_(2−x)Z′_(x)O_(7+/−y),where S denotes all cations (be it one cation species or more than one)of the secondary oxide, Z and Z′ respectively denote a cation of theprimary oxide, y is a number less than 1, and x is a number in the rangefrom about 0.01 to about 1.99. Here, the molar fraction of secondaryoxide cation is equivalent to the mole fraction of primary oxidecation—both have a value of 2 moles of cation per mole of material. Thevalue of y is dependent upon the tendency of the cations towardstetravalent states vs. trivalent states and will vary as required bycharge balancing. As discussed previously, FIG. 1 demonstrates anexample of the performance of such a material, where S denoteslanthanum, Z is cerium, and Z′ is hafnium.

Thermal barrier coatings made of cerates, such as lanthanum cerate, andhafnates, such as lanthanum hafnate (La₂Hf₂O₇) are known in the art.See, for example, U.S. Pat. No. 6,835,465 and U.S. Pat. No. 6,387,526.Although the compositions used for these coatings are similar to some ofthose described above, the coatings described in the art have markedlydifferent wetting resistance properties compared to the materials andarticles described herein. Thermal barrier coatings are generallyapplied using thermal spray techniques or physical vapor techniques,both of which are known to produce coatings having relatively highlevels of porosity. Typical industrial thermal barrier coatings haveporosity in the range from about 10 percent to about 25 percent, andresearchers have shown that thermal barrier coatings made with lanthanumcerate, for example, exhibit these typical porosity levels (see, forexample, Cao et. al, Advanced Materials, vol 15. issue 17, pp 1438-1442(2003)). The porosity is generally thought to provide an advantage inthese applications in that it may enhance the thermal resistance andstrain compliance of the coating. For instance, it is well documentedthat the strain compliance necessary for thermal cycling of thermalbarrier coatings requires significant amounts of porosity to beincorporated into the coatings, in the form of intercolumnar gaps inEB-PVD coatings and porosity between splats in thermal spray coatings.The distribution and morphology of coatings deposited by both processeshas been studied extensively to understand the enhancement in thermalresistance in the coatings as well as the detrimental effects on thermaland mechanical properties caused by the sintering loss of the porosity.Such work has determined that sintering of the coating results in adecrease in porosity and increase in Young's modulus, thereby resultingin higher thermally induced stresses and a decrease in thermal fatiguelifetime of the thermal barrier coating. For these reasons, thermalbarrier coatings generally are structured to maintain a high level ofporosity over long lifetimes at elevated temperatures.

In stark contrast to thermal barrier coatings, however, the oxidesapplied in certain embodiments of the present invention aresignificantly more dense, because their primary function is not toinhibit heat transfer to the substrate, but to inhibit buildup ofliquids, ice, or other foreign matter at the coating surface. The highporosity levels described in the thermal barrier coating arts generallywould not be suitable for use in many embodiments of the presentinvention. In fact, as noted above, in many heat transfer applicationsthe material is designed to minimize thermal resistance, which typicallywould require achieving porosity levels that are as low as practicallyattainable.

The novel properties described for the above embodiments lend themselvesto a host of useful applications where resistance to wetting by liquidsis desirable. A condenser used, for instance, to transfer heat between ahot vapor and a cooling fluid, such as is used in chemical processing,water desalination, and power generation, is an example of an embodimentof the present invention using the articles and materials describedabove. FIG. 6 illustrates one common type of condenser: the surfacecondenser 500. Steam, for example, enters shell 502 through inlet 504,whereupon it is condensed to water on the exterior surface ofcondensation tubes 506, through which flows a cooling fluid 508, such aswater. The material (not shown) described above is disposed on thisexterior surface of the condensation tubes 506, thereby promotingdropwise condensation of condensate water from the steam. The condensateis easily shed from the tubes 506 by the material and exits from shell502 via condensate outlet 510.

In certain applications, such as, for example, steam turbines, metalcomponents are subject to impinging drops of water as well as condensingdrops. As steam expands in a turbine, water droplets (typicallyfog-sized) appear in the flow stream. These droplets agglomerate on theturbine blades and other components and shed off as larger drops thatcan cause thermodynamic, aerodynamic, and erosion losses in turbines.The ability to shed water droplets from components before they have achance to agglomerate into substantially larger drops is thus importantto maximize system lifetime and operation efficiency. As noted above,many of the compositions applied in embodiments of the present inventionpromote dropwise condensation, so that liquid is shed from the surfacein small drops rather than in larger sheets. Accordingly, embodiments ofthe present invention include a steam turbine assembly comprising thearticle described above. In particular embodiments, the article is acomponent of a steam turbine assembly, such as a turbine blade, aturbine vane, or other component susceptible to impingement of waterdroplets during turbine operation.

Certain embodiments of the present invention may reduce the formation,adhesion, and/or accumulation of ice on surfaces. Icing takes place whena water droplet (sometimes supercooled) impinges upon the surface of anarticle, such as an aircraft component or a component of a turbineassembly (for example, a gas or wind turbine), and freezes on thesurface. The build-up of ice on aircraft, turbine components, and otherequipment exposed to the weather, increases safety risks and generatescosts for periodic ice removal operations. Certain embodiments of thepresent invention include an aircraft that comprises the articles andmaterials described above; a component of such an aircraft suitable toserve as the embodied article may include, for example, a wing, tail,fuselage, or an aircraft engine component. Non-limiting examples ofaircraft engine components that are suitable as articles in embodimentsof the present invention include the nacelle inlet lip, splitter leadingedge, booster inlet guide vanes, fan outlet guide vanes, sensors and/ortheir shields, and fan blades.

Icing is a significant problem for wind turbines, as the build-up of iceon various components such as anemometers and turbine blades reduces theefficiency and increases the safety risks of wind turbine operations.Wind turbine blades and other components are often made of lightweightcomposite materials such as fiberglass in order to save weight, and thebuild-up of ice can deleteriously load the blades to a point thatsignificantly reduces their effectiveness. In certain embodiments of thepresent invention, an article as described above is a component, such asa turbine blade, anemometer, gearbox, or other component, of a windturbine assembly.

As other components exposed to the weather are also adversely affectedby ice and/or water accumulation, other embodiments may include, forinstance, components of other items exposed to the weather, such aspower lines and antennas. The ability to resist wetting may benefit ahost of components that are so exposed, and the examples presentedherein should not be read as limiting embodiments of the presentinvention to only those named applications.

EXAMPLE

Without further elaboration, it is believed that one skilled in the artcan, using the description herein, utilize the present invention to itsfullest extent. The following example is included to provide additionalguidance to those skilled in the art in practicing the claimedinvention. The example provided is merely representative of the workthat contributes to the teaching of the present application.Accordingly, this example is not intended to limit the invention, asdefined in the appended claims, in any manner.

A coating in accordance with embodiments described herein was depositedon a commercially pure titanium substrate by radio frequency magnetronsputtering. The sputtering target was produced by pressing and sinteringa mixture of primary oxide, here cerium oxide, and secondary oxide, herelanthanum oxide, where the molar ratio of primary oxide cations to thetotal cations present was about 0.5. A coating with a thickness of about300 nm was produced using a deposition rate of 49 Å/min at a forwardpower of 100 watts in a 7% oxygen/93% argon gas mixture. The contactangle with water for the coating was about 113 degrees. The coatingexhibited dropwise condensation in steam.

While only certain features of the invention have been illustrated anddescribed herein, many modifications and changes will occur to thoseskilled in the art. It is, therefore, to be understood that the appendedclaims are intended to cover all such modifications and changes as fallwithin the true spirit of the invention.

1. An article, comprising: a coating having a surface connected porositycontent of up to about 5 percent by volume, wherein the coatingcomprises a material that comprises a primary oxide and a secondaryoxide, wherein (i) the primary oxide comprises a cation selected fromthe group consisting of cerium, praseodymium, terbium, and hafnium, and(ii) the secondary oxide comprises a cation selected from the groupconsisting of the rare earth elements, yttrium, and scandium.
 2. Thearticle of claim 1, wherein the coating has a surface connected porositycontent of up to about 2 percent by volume.
 3. The article of claim 2,wherein the material makes up at least about 50 percent by volume of thecoating.
 4. The article of claim 1, wherein the coating furthercomprises a surface texture.
 5. The article of claim 1, wherein theprimary oxide comprises a cation selected from the group consisting ofcerium and hafnium.
 6. The article of claim 1, wherein the secondaryoxide comprises a cation selected from the group consisting oflanthanum, praseodymium, and neodymium.
 7. The article of claim 1,wherein the primary oxide comprises cerium oxide.
 8. The article ofclaim 7, wherein the secondary oxide comprises lanthanum oxide.
 9. Thearticle of claim 8, wherein cerium cation makes up at least about 25molar percent of the cations present in the material.
 10. The article ofclaim 9, wherein cerium cation makes up from about 45 molar percent toabout 55 molar percent of the cations present in the material.
 11. Thearticle of claim 7, wherein the secondary oxide comprises praseodymiumoxide.
 12. The article of claim 11, wherein cerium cation makes up atleast about 7 molar percent of the cations present in the material. 13.The article of claim 12, wherein cerium cation makes up a molarpercentage of the cations present in the material in the range fromabout 7 percent to about 60 percent.
 14. The article of claim 7, whereinthe secondary oxide comprises neodymium oxide.
 15. The article of claim14, wherein cerium cation makes up at least about 20 molar percent ofthe cations present in the material.
 16. The article of claim 15,wherein cerium cation makes up a molar percentage of the cations presentin the material in the range from about 20 percent to about 50 percent.17. The article of claim 1, wherein the primary oxide comprises aplurality of oxides, each oxide comprising a cation selected from thegroup consisting of cerium, praseodymium, terbium, and hafnium.
 18. Thearticle of claim 17, wherein a ratio of primary oxide cation molarfraction to secondary oxide cation molar fraction is about
 1. 19. Thearticle of claim 18, wherein the material has a composition described bythe chemical formula S₂Z_(2−x)Z′_(x)O_(7+/−y), where S denotes allcations of the secondary oxide, Z and Z′ respectively denote a cation ofthe primary oxide, y is a number less than 1, and x is a number in therange from about 0.01 to about 1.99.
 20. The article of claim 19,wherein S comprises lanthanum, Z is cerium, and Z′ is hafnium.
 21. Thearticle of claim 1, wherein the substrate comprises a metal selectedfrom the group consisting of aluminum and its alloys, steel, stainlesssteel, nickel and its alloys, copper and its alloys, and titanium andits alloys.
 22. A condenser comprising the article of claim
 1. 23. Anaircraft comprising the article of claim
 1. 24. A turbine assemblycomprising the article of claim
 1. 25. The article of claim 1, whereinthe article is a component of a steam turbine assembly.
 26. An aircraftengine assembly comprising the article of claim
 1. 27. A wind turbineassembly comprising the article of claim
 1. 28. A method for protectingan article, comprising: depositing a coating on a substrate, wherein thecoating has a surface connected porosity content of up to about 5percent by volume, wherein the coating comprises a material thatcomprises a primary oxide and a secondary oxide, wherein (i) the primaryoxide comprises a cation selected from the group consisting of cerium,praseodymium, terbium, and hafnium, and (ii) the secondary oxidecomprises a cation selected from the group consisting of the rare earthelements, yttrium, and scandium.